The difficulties in meta-analysis in the social sciences vary from field to field -- psychology is qualitatively different from anthropology or history -- but if we had to generalize, they have more to do with the sheer dimensionality of the problem combined with the difficulty of precise measurement (and sometimes obtaining a large-enough sample), than with the field being "opinion based" or the existence of "mutually exclusive schools of thought" (I don't know how this differs from many other, more tractable research areas, and I don't see how this can be reconciled with saying that there's too much consensus).
You see similar social phenomena in other fields with a measurement problem, for example programming language research ...
Touché! :) (although there's plenty of social science research that is much more empirically rigorous than PL research, and the certainty/evidence ratio is certainly higher in PL)